dt-bindings: dmaengine: rcar-dmac: document R8A77965 support

Add documentation for r8a77965 compatible string to rcar-dmac device
tree bindings documentation.

Signed-off-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Rob Herring <robh@kernel.org>